title: Molecular structures of centromeric heterochromatin and karyotypic evolution in the Siamese crocodile (Crocodylus siamensis) (Crocodylidae, Crocodylia).
abstract: Crocodilians have several unique karyotypic features, such as small diploid chromosome numbers (30-42) and the absence of dot-shaped microchromosomes. Of the extant crocodilian species, the Siamese crocodile (Crocodylus siamensis) has no more than 2n = 30, comprising mostly bi-armed chromosomes with large centromeric heterochromatin blocks. To investigate the molecular structures of C-heterochromatin and genomic compartmentalization in the karyotype, characterized by the disappearance of tiny microchromosomes and reduced chromosome number, we performed molecular cloning of centromeric repetitive sequences and chromosome mapping of the 18S-28S rDNA and telomeric (TTAGGG)( n ) sequences. The centromeric heterochromatin was composed mainly of two repetitive sequence families whose characteristics were quite different. Two types of GC-rich CSI-HindIII family sequences, the 305 bp CSI-HindIII-S (G+C content, 61.3%) and 424 bp CSI-HindIII-M (63.1%), were localized to the intensely PI-stained centric regions of all chromosomes, except for chromosome 2 with PI-negative heterochromatin. The 94 bp CSI-DraI (G+C content, 48.9%) was tandem-arrayed satellite DNA and localized to chromosome 2 and four pairs of small-sized chromosomes. The chromosomal size-dependent genomic compartmentalization that is supposedly unique to the Archosauromorpha was probably lost in the crocodilian lineage with the disappearance of microchromosomes followed by the homogenization of centromeric repetitive sequences between chromosomes, except for chromosome 2.
authors.affiliation: Laboratory of Animal Cytogenetics, Biosystems Science Course, Graduate School of Life Science, Hokkaido University, Sapporo, 060-0810, Japan.
authors.forename: Taiki
authors.initials: T
authors.lastname: Kawagoshi
authors.forename: Chizuko
authors.initials: C
authors.lastname: Nishida
authors.forename: Hidetoshi
authors.initials: H
authors.lastname: Ota
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Kumazawa
authors.forename: Hideki
authors.initials: H
authors.lastname: Endo
authors.forename: Yoichi
authors.initials: Y
authors.lastname: Matsuda
mesh_terms: D000489:Alligators and Crocodiles / Q000235:genetics*; D000818:Animals; D001482:Base Composition; D001483:Base Sequence; D015139:Blotting, Southern; D002503:Centromere / Q000235:genetics*; D002874:Chromosome Mapping; D003001:Cloning, Molecular; D006570:Heterochromatin / Q000235:genetics*; D017404:In Situ Hybridization, Fluorescence; D007621:Karyotyping; D008969:Molecular Sequence Data; D012335:RNA, Ribosomal / Q000235:genetics; D017422:Sequence Analysis, DNA; D016615:Telomere / Q000235:genetics
chemical_list: D006570:Heterochromatin; D012335:RNA, Ribosomal